带连续管道的grep不起作用 [英] grep with continuous pipe does not work

问题描述

(也许是"tcpflow"问题)

(maybe it is the "tcpflow" problem)

我编写了一个脚本来监视HTTP流量，然后先安装tcpflow，然后安装grep

I write a script to monitoring http traffic, and I install tcpflow, then grep

它有效(并且您应该发出http请求，例如curl www.163.com)

it works (and you should make a http request, for example curl www.163.com)

sudo tcpflow -p -c -i eth0 port 80 2>/dev/null | grep '^Host: '

它连续输出这样的输出

Host: config.getsync.com
Host: i.stack.imgur.com
Host: www.gravatar.com
Host: www.gravatar.com

但是我不能继续使用管道

but I can't continue to use pipe

不起作用(无输出)

sudo tcpflow -p -c -i eth0 port 80 2>/dev/null | grep '^Host: ' | cut -b 7- 

不起作用(无输出)

sudo tcpflow -p -c -i eth0 port 80 2>/dev/null | grep '^Host: ' | grep H

当我将sudo tcpflow替换为cat foo.txt时，它会起作用:

When I replace sudo tcpflow with cat foo.txt, it works:

cat foo.txt | grep '^Host: ' | grep H

那么pipe或grep或tcpflow怎么了?

so what's wrong with pipe or grep or tcpflow ?

更新:

这是我的最终脚本: https://github.com /zhengkai/config/blob/master/script/monitor_outgoing_http.sh

推荐答案

我认为问题是由于 stdio buffering ，您需要在调用grep之前使用GNU stdbuf，

I think the problem is because of stdio buffering, you need to use GNU stdbuf before calling grep,

sudo tcpflow -p -c -i eth0 port 80 2>/dev/null | stdbuf -o0 grep '^Host: '

对于-o0，基本上意味着tcpflow的输出(stdout)流将不被缓冲.默认行为是在发送到管道中的下一个命令之前，自动将数据缓冲到4096 ¹ 个字节块中，这是使用stdbuf

With the -o0, it basically means the output (stdout) stream from tcpflow will be unbuffered. The default behavior will be to automatically buffer up data into 4096¹ byte chunks before sending to next command in pipeline, which is what overriden using stdbuf

_{1.将此很好的细节纳入主题.}

_{1. Refer this nice detail into the subject.}

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